1. Field of the Invention
The present invention relates to controlling delivery of power to electronic circuitry and, more particularly, to controlling delivery of power to thermal print head elements to improve print output quality.
2. Related Art
Thermal printers typically contain a linear array of heating elements (also referred to herein as xe2x80x9cprint head elementsxe2x80x9d) that print pixels on an output medium by transferring pigment from a donor sheet to the output medium (such as plain paper). Each of the print head elements, when activated, transfers pigment to a region of the output medium passing underneath the print head element, creating what is referred to herein as a xe2x80x9cspot.xe2x80x9d Digital images are rendered as two-dimensional arrays of very small and closely-spaced spots.
Different numbers and combinations of print head elements may be active at different times when printing a digital image, depending on the intensities of the pixels in the digital image. As a result of the circuitry that is typically used to provide power to the print head elements in a thermal printer, spots that are printed by a large number of contemporaneously active print head elements appear lighter than spots that are printed by a small number of contemporaneously active print head elements. This difference in rendered intensity is undesirable because it corresponds to the number of contemporaneously active print head elements, rather than to the intensities of the pixels in the source image being printed. The result is a printed image having undesired variations in intensity that do not accurately reflect the intensities of the pixels in the source image being printed.
One attempt to solve this problem has been to increase the gray levels of pixels in a particular row of a grayscale digital image being printed as the aggregate gray level of the pixels in the row increases. For example, if the aggregate gray level of the pixels in a row is large, the gray level of each pixel may be increased in an attempt to compensate for the effective decrease in gray level described above. The gray level of a pixel is typically increased by activating the corresponding print head element for a greater number of print head cycles, thereby printing a greater number of spots than would normally be used to print the pixel. Although this technique may result in some improvement in output image quality, it may fail to work properly in conjunction with certain conventional techniques used in thermal printing, as described in more detail below.
What is needed, therefore, are improved techniques for accurately printing different tones (e.g., gray levels) using a thermal printer, regardless of the number of print head elements that are contemporaneously active at any particular point in time.
In one aspect of the present invention, a method is provided for providing the same amount of energy to each print head element in a thermal printer during each print head cycle used to print an image, regardless of the number of print head elements that are active during each print head cycle. In one embodiment, the desired amount of energy is provided to a plurality of print head elements that are active during a print head cycle by delivering power to the plurality of print head elements for a period of time whose duration is based in part on the number of active print head elements. The period of time may be a portion of the print head cycle. For example, the number of print head elements that are to be active during a particular print head cycle may be determined (e.g., at or slightly before the beginning of the print head cycle), and power may be delivered to the active print head elements for an amount of time during the print head cycle based on the number of active print head elements. The amount of time may be chosen so that the total amount of energy delivered by each active print head element to an output medium during each print head cycle remains constant from print head cycle to print head cycle, regardless of the number of active print head elements in any particular print head cycle.
A correction factor may be used in the process of selecting the amount of time to activate print head elements during a particular print head cycle. In one aspect of the present invention, a parameter of the correction factor (or an approximation thereto) may be developed using a source target rendered on an output medium as an output target. The output target may be visually inspected and the value of the parameter may be derived from observations made during the visual inspection. For example, as described in more detail below, the source target may contain a first and second plurality of source regions having the same intensity (e.g., gray level). Pixels in the first plurality of source regions are arranged so that a first predetermined number of heating elements are active when the first plurality of source regions are rendered on the output medium as a first plurality of output regions. The first plurality of source regions are rendered on the output medium using a constant duty cycle. Pixels in the second plurality of source regions are arranged so that a second predetermined number of heating elements are active when the second plurality of source regions are rendered on the output medium as a second plurality of output regions. The second plurality of source regions are rendered on the output medium using a plurality of duty cycles (e.g., as described below with respect to steps 708 and 728). The second plurality of output regions therefore have a variety of blacknesses.
The output target may be visually inspected to identify one of the second plurality of output regions whose blackness most closely matches the blackness of the first plurality of output regions. The second plurality of output regions may be located near the first plurality of output regions to facilitate such identification. The parameter of the correction factor may be determined based on the selected one of the second plurality of output regions, as described in more detail below.
Additional aspects and embodiments of the present invention will be described in more detail below.